Add a pre- and post-execution hooks

[krassowski]

Problem

• When the notebook is closed the metadata for cell timing are not updated
• When the notebook is closed and execution of a long-running cell completes, there is no way for extensions to offer notifications (e.g. sending an email)

Proposed Solution

Allow extensions to register pre- and post-execution functions to run which would be allowed to populate the cell metadata.

Maybe special-case execution timing as a hook that is available by default, and enabled based on an argument passed to the JSON API?

Additional context

The timing metadata population is implemented in JupyterLab here:

https://github.com/jupyterlab/jupyterlab/blob/ef5a055976e02fee6c4d877a6a6df756f3323f79/packages/cells/src/widget.ts#L1736-L1787

I see there is a TODO note in this repository about it:

jupyter-server-nbmodel/src/executor.ts

Line 122 in da00ed8

// FIXME quid of deletedCells and timing record